Nuclear security applications often require differentiation of a threat source from ambient background noise.
Some nuclear security applications involve detecting and monitoring radiation sources which move relative to the detector. Examples include radiation portal monitors at sea ports and border crossings that screen cargo containers, vehicles, or pedestrians for radiological or nuclear materials and mobile radiation detectors deployed in search operations.
If separate measurements are made with and without the threat source being present, then the presence of the threat source can be differentiated by background subtraction. However, independent background measurements are not always practical or even possible in nuclear security scenarios. Moreover, if they are possible, the variance of the background measurement increases with the intensity of the background.
Efforts have been directed at either physically reducing the background or differentiating the response of a radiation detector to the background from that produced by a threat source. Physical reduction of the background can be accomplished by shielding or collimation of the detectors or by preparation of the site where the detectors are placed to remove radioactive materials contributing to the background. But while collimation reduces background noise, it also reduces both the solid angle from which the detectors can observe radiation and the time during which a moving source can be observed.
Previously, a number of approaches for differentiating the background from threat sources have been implemented, including spectroscopy (energy differentiation), imaging (angular differentiation), and localization (spatial differentiation).
In some nuclear applications, multiple detectors are present and observing the same moving source. The response of multiple detectors can be summed to achieve an improved sensitivity for detection of a threat source. In the case of identical detectors observing the same source in the same geometrical configuration, the ratio of the response to the threat source to the uncertainty in the background increases only as fast as the square root of the number of detectors. As such, summation of detector responses is perceived as having drawbacks.
Work by others has considered a time series evaluation of the detector response in order to identify the unique time profile of a radiation detector response to a moving point source.
A need exists in the art for a method and a system for the detection of radioactive sources that are in motion. This method should maximize the sensitivity of the detectors to the presence and the motion of the sources under consideration.